There has been a growing strong demand for flame retardant resin and rubber. However, there is a strong request to limit to a safe method by employing metallic hydroxide material as a means of making them flame retardant rather than using the current common method by a halogen type flame retardant material which simultaneously uses halogen and antimony trioxide. However, even magnesium hydroxide which has the fewest drawbacks among the metallic hydroxide materials requires a rather large amount of combination of about 70 weight % with respect to about 30 weight % of the resin and/or rubber to reach the targeted level of flame retardancy. Therefore, there is a problem caused in that an unacceptable degree of loss in some characteristics of the resin and rubber.
At this point, one of the inventors proposed a magnesium hydroxide solid solution having divalent transition metal represented as the general formula (1); EQU Mg.sub.1-x M.sup.2+ .sub.x (OH).sub.2 ( 1)
wherein M.sup.2+ denotes at least one type of divalent metal selected from the group consisting of Mn.sup.2+, Fe.sup.2+, Co.sup.2+, Ni.sup.2+, Cu.sup.2+ and Zn.sup.2+, and x indicates a range of 0.001.ltoreq.x.ltoreq.0.9. When this magnesium hydroxide solid solutions is compounded in the resin and/or rubber, flame retardancy and acid resistance are greatly improved, and as a result, the targeted level of flame retardancy can be reached with a minimum amount smaller than that of conventional magnesium hydroxide.
However, as a result of further research, several problems to be solved for the magnesium hydroxide solid solution shown by the general formula (1) were found. The first problem was that compared with the magnesium raw material, the M.sup.2+ material is expensive, in particular, there is a problem of Co and Ni being expensive. For example, for the Mg.sub.1-x Ni.sub.x (OH).sub.2 type magnesium hydroxide solid solution with a range of x being 0.2.ltoreq.x.ltoreq.0.6, the flame retardancy and the acid resistance clearly show improvement compared with magnesium hydroxide. However, the solid solution containing just that amount of Ni, is very expensive compared with magnesium hydroxide. The second problem was that along with the increase of the M.sup.2+ contained, the crystal growth of the magnesium hydroxide solid solution and the dispersibility become worse. However, such a level differs according to the type of the M.sup.2+ contained.
The present inventors carefully conducted research to resolve the above mentioned issues. As a result, the reason that the magnesium hydroxide solid solution showed superior flame retardancy compared with magnesium hydroxide was discovered. This is due to the catalytic effects of the M.sup.2+ contained and the lower starting temperature of the dehydration compared with that of the magnesium hydroxide. Further, it was found out that the catalytic effects contribute to improvement in flame retardancy compared with the lower starting temperature of dehydration.
At this point, at least for the catalytic effects of the M.sup.2+, it was surmised that just the M.sup.2+ exposed on the surface of the magnesium hydroxide solid solution in contact with the resin or the rubber of having the possibility to contact this, contributed to this effect. Therefore, if it were possible to place M.sup.2+ selectively in the vicinity of the surface of the magnesium hydroxide crystals, compared with when the M.sup.2+ is distributed homogeneously in the magnesium hydroxide crystals, it was thought that it would be possible to work the same catalytic effect with an extremely smaller amount of M.sup.2+ compared to when the M.sup.2+ is distributed homogeneously. Furthermore, in the vicinity of the surface of the crystals of the magnesium hydroxide solid solution wherein the M.sup.2+ is unevenly distributed with a high concentration in the vicinity of the surface of the magnesium hydroxide, the amount of the crystal water dehydrating at a low temperature is said to be smaller than in the case when the M.sup.2+ is distributed evenly over the entire surface of the crystals in a high concentration because a dehydration of the crystal water is supposed to take place at a temperature lower than that for the magnesium hydroxide. However, this effect is supposed to contribute to flame retardancy to some degree.